The present invention relates to a set of provisional prosthesis instrumentation, and more particularly to instrumentation which is suitable for use in hip hemiarthroplastly procedures. In such procedures, the proximal portion of the femur is replaced with a suitable prosthetic hip joint implant or implant assembly which will mate or articulate directly with the natural acetabulum (as opposed to mating with a prosthetic acetabular joint implant component).
The present invention also relates to an articulated prosthesis comprising a first articulating portion having a part-spherical bearing surface for engaging the acetabulum and a second inner part-spherical articulation surface and a second articulating component having an outer part-spherical bearing surface adapted to engage the second inner part-spherical bearing surface of the first component. More particularly, the invention relates to a prosthetic hip joint comprising a component for contacting the acetabulum, including the part-spherical inner and outer bearing surfaces and a femoral component having a part-spherical head.
Artificial implants, including hip joints, shoulder joints and knee joints, are widely used in orthopedic surgery. Hip joint prostheses are the most common. The human hip joint acts mechanically as a ball and socket joint, wherein the ball-shaped head of the femur is positioned within the socket-shaped acetabulum of the pelvis. Various degenerative diseases and injuries may require replacement of all or a portion of a hip using synthetic materials. Prosthetic components are generally made from either metals, ceramics, or plastics.
Total hip arthroplasty and hemi-arthroplasty are two procedures well known within the medical industry for replacing all or part of a patient's hip. A total hip arthroplasty replaces both the femoral component and the acetabular surface of the joint, so that both a femoral prosthesis and an acetabular prosthesis are required. A conventional acetabular prosthesis may include a metal cup, a metal cup and a polyethylene liner, or in some cases only a polyethylene cup, all of which may be formed in various shapes and sizes. Generally, a metal cup and a polymeric liner are used. The liner may be made of a variety of materials, including ultra high molecular weight polyethylene and ceramic materials. The metal cup is usually of generally hemispherical shape and features an outer, convex surface and an inner, concave surface that is adapted to receive a cup liner. The liner fits inside the cup and has a convex outer and concave inner surface. The cup liner is the bearing element in the acetabular component assembly which engages the femoral component. The convex outer surface of the liner corresponds to the inner concave surface of the metal cup or the natural acetabulum, and the liner concave surface receives the part-spherical head of a femoral component.
In general, a bipolar prosthesis has an external surface which articulates with the natural acetabulum and an internal surface which articulates with the part-spherical head member of a prosthetic femoral component. Another type of prosthesis is referred to as a unipolar endoprosthesis in which the prosthetic femoral component includes a spherical head member which is large enough to articulate directly with the natural acetabulum. Both of the above hemiarthroplasty procedures enable articulation with the natural acetabulum. These two procedures permit later conversion to a total hip replacement in which the acetabular portion is also replaced with a prosthetic acetabular component. With the bipolar conversion procedure, the bipolar prosthesis is removed from the head of the hip stem, and an acetabular prosthesis implanted which mates with the head of the remaining femoral component. With the unipolar endoprosthesis, these may typically be modular (where the head is a separate component from the stem portion of the hip prosthesis). This permits the larger unipolar head to be removed and replaced with a smaller prosthetic femoral head to mate with an acetabular prosthetic implant component.
A number of methods are known for retaining the ball or part-spherical femoral head in the bearing surface of the acetabulum or socket. In the most common method, referred to herein as “semi-constrained” construction, the patient's own anatomy, i.e., his muscles, tendons and ligaments, are used to retain the ball within the socket. For this construction, a hemispherical socket typically is used which allows the ball and its attached arm, such as the neck of a femoral component, the maximum amount of movement without contact of the arm with the edge of the socket. The surgeon, when installing such a semi-constrained joint, aligns the ball and socket as closely as possible with the patient's natural anatomy so that the patient's movements do not tend to dislocate the ball from the joint. This requires precise alignment and in some cases the precise alignment is difficult to obtain. This is especially true when the primary or first artificial joint such as a prosthetic hip implantation has to be revised with the implantation of a second or revision implant.
In order to increase the inherent stability against dislocation of such semi-constrained constructions, it has become conventional to add a cylindrical portion to the hemispherical socket to make it deeper. Although the ball is not physically constrained by the socket, by this adjustment the ball does have further to travel than if just a hemisphere had been used and thus some reduction in the propensity towards dislocation is achieved. Ball and socket joints of this type generally provide an arc or range-of-motion of approximately 115° when a 28 mm diameter sphere is used for the head of, for example, the femoral component and then the socket or bearing component is made a few millimeters deeper than a hemisphere. Larger ranges-of-motions can be obtained by keeping the size of the neck or arm attached to the ball constant and increasing the diameter of the ball. In this way, the angular extent of the arm relative to the ball becomes smaller. In the limit, if the ball could be made progressively larger and larger, a range-of-motion of 180° could be achieved.
One problem with artificial joint implants, such as hip implants, is that there is a very small percentage of hip joint implant procedures using the semi-constrained construction which dislocate. Such dislocations essentially make the patient immobile and can necessitate a second operation.
An alternative to the semi-constrained construction is a construction wherein the ball is physically constrained within the socket. In this construction, the part-spherical acetabular cup includes an extension which extends beyond the equator of the cup. Since the bearing encompasses more than one-half of the ball, the bearing thus constrains the ball from dislocation. Alternately, if the bearing is made of plastic, such as ultra-high molecular weight polyethylene (UHMPWE), the ball and bearing may be assembled by forcing the bearing over the ball. Obviously, the more the ball is surrounded by the bearing, i.e. the greater the latitude below the equator which the bearing extends, the more difficult it is to insert the head or ball of the ball and socket joint. In addition, the more that the bearing encompasses the ball, the smaller the range-of-motion for the ball prior to contact of the bearing with the neck or arm attached to the ball.
U.S. Pat. No. 4,960,427 to Douglas G. Noiles, relates to a constrained ball and socket joint for implantation in the hip. The design requires a multitude of parts in order to accomplish the constrained relationship between the ball of the femoral component and the bearing socket of the acetabular component.
U.S. Pat. No. 4,770,661 to Indong Oh, relates to a cup assembly having an insert or bearing within the cup and a locking ring at least partially within the cup. The locking ring has a conical surface that mates with the conical surface of the head with the assembly held together by fastener means, such as screw threads, on the locking rings and the cup.
U.S. Pat. No. 5,062,823 to Mark Forte, relates to a prosthetic hip joint for including an acetabular cup with a split ring for retaining the femoral head in the bearing. The split ring includes an upper split locking ring with a section for sliding within a guide in the bearing of the acetabular component. After assembly, the locking ring slides distally to lock the head of the femoral component in the acetabular cup.
U.S. Pat. No. 5,156,626 to Broderick et al., relates to instrumentation to be utilized with a trial bipolar hip prosthesis. The instrumentation includes two inner components to be used with an outer hemispherical bearing component with one component allowing rotation of a femoral head within the inner component, thereby producing a bipolar hip implant and wherein the second inner component allows the outer bearing component to be rotationally fixed with respect to the femoral component, thereby producing a unipolar hip implant. A plurality of sizes for the three components may be provided.
There has also been a long felt need to provide a means of converting a trial or provisional bipolar prosthesis into a unipolar trial prosthesis. Such a trial system includes an inner component and an outer component with the inner component comprising a substantially hemispherical exterior surface adapted to operatively engage a substantially hemispherical inner surface on the outer component. If the components are to be used in a hip joint, the outer component has a substantially hemispherical exterior surface for engaging the acetabulum. The inner component is connected to or integral with, in the case of a hip arthroplasty, a femoral component at its proximal end. Normally, this head portion is the inner component of the provisional or trial system.
In use, the trial can act as a bipolar hip prosthesis in which the inner component articulates with respect to the outer component and the outer component articulates with respect to the acetabulum.
As shown in U.S. Pat. No. 5,156,626 a second inner component is necessary to convert the trial bipolar prosthesis to a trial unipolar prosthesis. The inventor herein has developed a locking element which can be inserted into aligned grooves in the inner and outer components, thus changing the bipolar prosthesis into a unipolar prosthesis since pins of the locking element can engage the aligned grooves and prevent the relative rotation of the outer component with respect to the inner component.
The system of the present invention can be designed to eliminate the necessity of removing a modular head from, for example, a femoral component and replacing it with a modular head of a different design such as shown in U.S. Pat. No. 5,156,626.
In addition, it is desirable to preclude the trial head or inner component from disengaging from the trial outer component during this procedure. To accomplish this, a recessed portion or flattened portion is located intermediate spherical portions of the part-spherical inner component with the spherical inner surface of the outer component extending beyond the equator thereof to a diameter at its open end less than the diameter of the outer circumference of the part-spherical surfaces of the inner component or head but larger than the recessed portion so that the recessed area of the inner component or head may be aligned with the smaller outer component diameter and passed therethrough during insertion. The flattened portion may be oriented on the head at angles anywhere between perpendicular and coaxial with the polar axis of the inner component. When the inner component or head is rotated so that a conical recess therein may be attached to a conical trunnion on, for example, a femoral component, and the recessed or flattened area on the head circumference is at an angle wherein it can no longer be aligned with the open end diameter of the outer component, the trial head is captured within the assembly. In this embodiment, the head can still include grooves which can be aligned with grooves in the outer component so the assembly can be converted from a bipolar hip prosthesis to a unipolar hip prosthesis.
While the constraining system described herein is taught in connection with a trial prosthesis, such could be utilized in an actual prosthetic implant where it is desirable to ensure that the head does not disengage from the acetabular component. The implant can be of any type used to replace a ball and socket joint such as a hip or shoulder.